Acid-Induced Room Temperature RAFT Polymerization: Synthesis and Mechanistic Insights

An acid-induced cyclohexanone/tert-butylhydroperoxide initiation system for ambient temperature reversible addition-fragmentation transfer (RAFT) polymerization of vinyl monomers is presented. The reaction system is optimized for the synthesis of poly(n-butyl acrylate) of various chain length. The polymerization shows typical living characteristics and polymers with dispersities close to 1.1 are obtained. Analysis of the polymer end groups by means of soft ionization mass spectrometry reveals the typical distribution of polymer containing both R and Z RAFT end groups and a minor distribution of a RAFT polymer carrying a cyclohexanone end group in α position. This observation demonstrates that the polymerization is initiated solely by ketone radicals despite a relatively complex initiation mechanism that involves several intermediates. The room temperature-derived homopolymers are successfully chain extended with tert-butyl acrylate resulting in well-defined block copolymer structures. To demonstrate the versatility of the approach, the room temperature RAFT polymerization is also applied to synthesize styrene and N-isopropylacrylamide, yielding best results for polystyrene. Finally, also a bisperoxide structure is tested as an alternative for the ketone/peroxide mixture. Polymerization proceeds substantially faster in this case and successful controlled polymerization to full conversion is achieved even at 0 °C. In general the proposed room temperature RAFT technique is very easy to carry out, in principle easily up scalable, metal free and shows high potential toward the synthesis of well-defined temperature sensitive materials.